stm32_serial: fix freezing serial port. Serial interrupt enable/disable...

stm32_serial: fix freezing serial port.  Serial interrupt enable/disable functions do not disable interrupts and can freeze device when serial interrupt is received while execution is at those functions.

Trivially triggered with two or more threads write to regular syslog stream and to emergency stream. In this case, freeze happens because of mismatch of priv->ie (TXEIE == 0) and actually enabled interrupts in USART registers (TXEIE == 1), which leads to unhandled TXE interrupt
and causes interrupt storm for USART.